Assessing the impact of street lighting on Platanus x acerifolia phenology

Autumn phenology is an important part of the tree growing season that is still poorly understood. In addition to the environmental factors that might affect its timing, there are artificial effects introduced by modern society that could interfere with it, such as the increasing use of artificial light to illuminate urban nights. This study investigates the relationship between outdoor public lighting and leaf senescence of Platanus x acerifolia that constitutes with more than 4000 individuals, and 6% of public greening in Florence, Italy. The difference in autumn phenology under two lighting conditions was assessed by analysing data collected in a real context, using a presence-absence protocol of green leaves on 283 trees during leaf fall season from 2014 to 2017. Trees were classified in two groups of different light exposure. In 2016-2017, data were also collected at Cascine park, the main green area within the city and darker than the monitored sites. According to the analysis, the percentage of trees with green leaves under luminaires was significantly higher than trees far from the luminaires, for all sites from mid-December to the end of January, and this effect was enhanced during 2016-2017 which was characterised by a colder winter. In the same year, the period of absence of green leaves at Cascine started at least 20 days earlier than the other sites. These findings should be taken into consideration by scientists because artificial light could affect autumn phenology and therefore the length of the vegetative season, and by urban greening and light managers during the design and management of public green spaces. Moreover, the presence-absence protocol proved to be suitable for collecting observations because it was easy to perform in a real context.